Liquid crystal display device and electronic device having the same

ABSTRACT

A liquid crystal display device includes an illuminator and a liquid crystal panel for performing displaying by using light which is emitted from the illuminator. The liquid crystal panel includes a pair of substrates and a liquid crystal layer provided therebetween. The liquid crystal layer is formed of a liquid crystal material which contains molecules having at least one of a carbon-carbon triple bond and a polycyclic group. The illuminator includes a light source causing primary generation of at least blue light, among other light which is used for displaying.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device, and inparticular, to a liquid crystal display device including a liquidcrystal layer which is formed of a low-viscosity liquid crystalmaterial. Moreover, the present invention also relates to an electronicapparatus incorporating such a liquid crystal display device.

2. Description of the Related Art

In recent years, the needs for displaying moving picture information ona liquid crystal display device are rapidly increasing. In order todisplay moving pictures on a liquid crystal display device with a highquality, it is necessary to reduce the response time (i.e., increase theresponse speed) of the liquid crystal layer, and it is a requirement toreach a predetermined gray scale level within one vertical scanningperiod (which typically is one frame).

As one technique for improving the response characteristics of a liquidcrystal display device, a technique of using a low-viscosity liquidcrystal material has been proposed. Low-viscosity liquid crystalmaterials are disclosed in Japanese Laid-Open Patent Publication No.10-292173 (Patent Document 1), for example.

However, conventional liquid crystal display devices have a problem inthat, when a low-viscosity liquid crystal material is used, the voltageretention rate may be lowered during use, thus resulting in displayunevenness. Therefore, liquid crystal display devices using alow-viscosity liquid crystal material lack in reliability, and have notbeen put to practical use.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention solve the above-describedproblems, and improve the reliability of a liquid crystal display deviceincluding a liquid crystal layer which is formed of a low-viscosityliquid crystal material.

A liquid crystal display device according to a preferred embodiment ofthe present invention is a liquid crystal display device including anilluminator and a liquid crystal panel for performing displaying byusing light which is emitted from the illuminator, wherein, the liquidcrystal panel includes a pair of substrates and a liquid crystal layerprovided between the pair of substrates; the liquid crystal layer isformed of a liquid crystal material which includes molecules having atleast one of a carbon-carbon triple bond and a polycyclic group; and theilluminator includes a light source causing primary generation of atleast blue light, among other light which is used for displaying.

In a preferred embodiment, a coefficient of rotational viscosity γ₁ ofthe liquid crystal material at approximately 20° C. is preferably about120 mPa·s or less.

In a preferred embodiment, the molecules contained in the liquid crystalmaterial have a chemical structure expressed by one of the followingformulae:

(where n in the formulae is an integer equal to or greater than 2; andany hydrogen atom contained in a ring structure in the formulae may be,independently, substituted by a halogen atom, a cyano group, or anisocyano group).

In a preferred embodiment, the liquid crystal material preferablycontains about 25 weight % or more of the molecules having the abovechemical structure.

In a preferred embodiment, a spectrum of blue light which is emitted bythe light source has a peak wavelength at about 380 nm or more.

In a preferred embodiment, the light source generates substantially nolight in an ultraviolet region.

In a preferred embodiment, the light source is a light-emitting diode.

In a preferred embodiment, the light source is an electroluminescenceelement.

In a preferred embodiment, the light source is a discharge tube.

In a preferred embodiment, the liquid crystal panel performs displayingin a vertical alignment mode.

In a preferred embodiment, the liquid crystal panel performs displayingin an in-plane switching mode.

In a preferred embodiment, the liquid crystal panel further includes aplurality of pixel regions each capable of modulating light emitted fromthe illuminator, and a switching element provided in each of theplurality of pixel regions.

An electronic apparatus according to a preferred embodiment of thepresent invention preferably is a liquid crystal display device havingthe above construction.

In a preferred embodiment, an electronic apparatus according to apreferred embodiment of the present invention further includes circuitryfor receiving a television broadcast.

An illuminator of a liquid crystal display device according to apreferred embodiment of the present invention includes a light sourcecausing primary generation of at least blue light, among other lightwhich is used for displaying, and therefore decomposition of themolecules contained in the liquid crystal material due to ultravioletlight is prevented and minimized. As a result, according to a preferredembodiment of the present invention, the reliability of a liquid crystaldisplay device including a liquid crystal layer which is formed of alow-viscosity liquid crystal material can be improved, and thus a liquidcrystal display device which is capable of performing high-qualitydisplaying for long hours can be provided.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically showing a liquid crystaldisplay device according to a preferred embodiment of the presentinvention.

FIG. 2 is a cross-sectional view schematically showing a liquid crystaldisplay device according to a preferred embodiment of the presentinvention.

FIG. 3 is an upper plan view schematically showing an active matrixsubstrate which is used for a VA mode liquid crystal display device.

FIG. 4 is a diagram schematically showing a relationship between pretiltdirections imparted to alignment films and a tilting direction of aliquid crystal molecule.

FIG. 5 is an upper plan view schematically showing an active matrixsubstrate which is used for an IPS mode liquid crystal display device.

FIG. 6 is an upper plan view schematically showing an active matrixsubstrate which is used for an IPS mode liquid crystal display device.

FIG. 7 is a graph showing an emission spectrum of blue LED #1 used for aprototype liquid crystal display device.

FIG. 8 is a graph showing an emission spectrum of blue LED #2 used for aprototype liquid crystal display device.

FIG. 9 is a graph showing an emission spectrum of blue LED #3 used for aprototype liquid crystal display device.

FIG. 10 is a graph showing emission spectrum of blue LED #4 used for aprototype liquid crystal display device.

FIGS. 11 (a) and (b) are graphs showing an emission spectrum of acold-cathode tube (CCFL) used for a liquid crystal display device of acomparative example.

FIG. 12 is a graph showing a voltage-transmittance curve of a VA modeliquid crystal display device.

FIG. 13 is a graph showing a voltage-transmittance curve of a VA modeliquid crystal display device, where transmittance is shown in logarithmon the vertical axis.

FIG. 14 is a graph showing a voltage-transmittance curve of a TN modeliquid crystal display device.

FIG. 15 is a graph showing a voltage-transmittance curve of a TN modeliquid crystal display device, where transmittance is shown in logarithmon the vertical axis.

FIG. 16 is a graph showing an absorption spectrum of a TAC filmcontaining an ultraviolet absorber.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The inventor has conducted a detailed analysis of the causes for theaforementioned problems occurring in a liquid crystal display device inwhich a low-viscosity liquid crystal material is used.

Being a non-emission type display device, a liquid crystal displaydevice includes an illuminator, and display is performed by modulatingthe light from the illuminator with a liquid crystal panel. As will bedescribed later, the inventor has ascertained that a minute amount ofultraviolet light is emitted from the illuminator. Furthermore, theinventor has ascertained that a liquid crystal material having a lowviscosity is likely to contain molecules which are susceptible todecomposition by ultraviolet light (i.e., unstable against ultravioletlight), and found that orientation disturbances and a decrease in thevoltage retention rate occur when such molecules are decomposed by theultraviolet light from the light sources.

In the illuminator of a commonly-used liquid crystal display device, acold-cathode tube is used as a light source. In the cold-cathode tube,mercury which is enclosed within the tube is excited by discharging togenerate ultraviolet light, and this ultraviolet light excites aphosphor which is enclosed in the tube, whereby visible light that isused for displaying (which typically is light containing red, green, andblue light) is generated. In other words, the cold-cathode tube causesprimary generation of ultraviolet light, and the ultraviolet lightcauses secondary generation of visible light.

Not all of the ultraviolet light that is generated from the mercury isused for exciting the phosphor, but a portion thereof is emitted outsidethe tube and reaches the liquid crystal panel. Although the ultravioletlight emitted outside the tube is so minute that it can hardly bedetected with a commonly-used illuminometer, the ultraviolet light willirradiate the liquid crystal panel for long periods of time, thuspromoting decomposition of the molecules in the liquid crystal materialand causing the aforementioned problems.

In recent years, liquid crystal display devices have come to be used forliquid crystal television sets which display images from a televisionbroadcast. It is contemplated that a liquid crystal television set willbe placed in a living room or the like, and used for very long hours.Therefore, a liquid crystal television set is required to have areliability such that it is capable of performing stable displaying forabout forty thousand hours (10 hours/day×365 days×10 years). In suchlong hours of use, decomposition of molecules due to ultraviolet lightfrom the illuminator presents a major problem.

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the drawings. Note that the presentinvention is not to be limited to the preferred embodiments below.

FIG. 1 shows a liquid crystal display device 100 according to thepresent preferred embodiment. The liquid crystal display device 100preferably includes an illuminator 10A and a liquid crystal panel 20which performs displaying by using light emitted from the illuminator10A. A diffusion sheet 30 for diffusing the light from the illuminator10A is provided between the illuminator 10A and the liquid crystal panel20.

The liquid crystal panel 20 preferably includes: a pair of substrates(e.g., glass substrates) 20 a and 20 b; a liquid crystal layer 21provided therebetween; and a pair of alignment films 22 a and 22 b whichare provided on the sides of the pair of substrates 20 a and 20 b facingthe liquid crystal layer 21. Although not shown in the figure,electrodes for applying a voltage across the liquid crystal layer 21 areformed on the substrates 20 a and 20 b.

The illuminator 10A is an LED array which includes a plurality oflight-emitting diodes (LEDs) arranged in a matrix array as lightsources. Specifically, the illuminator 10A includes red LEDs 12R, greenLEDs 12G, and blue LEDs 12B.

Through recombination of electrons and holes occurring at a pn junctionwhere a bias voltage is applied in the forward direction, the red LEDs12R, green LEDs 12G, and blue LEDs 12B generate red light, green light,and blue light, respectively. In other words, the red LEDs 12R, greenLEDs 12G, and blue LEDs 12B cause primary generation of red light, greenlight, and blue light, respectively; and white light which containsthese kinds of light is radiated onto the liquid crystal panel 20 so asto be used for color displaying.

As described above, the illuminator 10A of the liquid crystal displaydevice 100 includes light sources causing primary (i.e., direct)generation of light to be used for displaying, and thereforedecomposition of molecules due to ultraviolet light is unlikely to occureven when the liquid crystal layer 21 is formed of a low-viscosityliquid crystal material in order to improve the responsecharacteristics. Hence, orientation disturbances and decrease in thevoltage retention rate due to decomposition of the molecules in theliquid crystal layer are unlikely to occur, so that high-qualitydisplaying can be performed for long periods of time.

Although FIG. 1 illustrates an illuminator 10A which includes the redLEDs 12R, green LEDs 12G, and blue LEDs 12B, the present invention isnot to be limited thereto. An illuminator that includes blue LEDs andphosphors which absorb light from the blue LEDs and generate light inlonger wavelength regions may also be used. For example, an illuminatorthat includes blue LEDs and red LEDs as well as green phosphors whichgenerate green light by absorbing blue light, or an illuminator thatincludes blue LEDs, green phosphors, and red phosphors which generatered light by absorbing blue light may be used. By using an illuminatorthat includes light sources causing primary generation of at least bluelight, among other light which is used for displaying, decomposition ofthe molecules in the liquid crystal layer can be suppressed.

Moreover, although the illuminator 10A shown in FIG. 1 is a direct-typeilluminator in which LEDs are arranged in a matrix array immediatelyunder the liquid crystal panel 20, any other type of illuminator may beused. For example, as in an illuminator 10B shown in FIG. 2, it may be asidelight-type illuminator in which an LED 12 is disposed at an endsurface of a light guide plate 14 that is provided at the rear surfaceside of a liquid crystal panel 20 and in which light from the LED 12 isguided by the light guide plate 14 into the liquid crystal panel 20.

Preferred embodiments of the present invention can be suitably used forliquid crystal display devices of various display modes. For example, itmay be suitably used for a liquid crystal display device of a twistednematic (TN) mode, a vertical alignment (VA) mode, or an in-planeswitching (IPS) mode.

Now, a VA mode liquid crystal display device will be described. FIG. 3schematically shows an active matrix substrate 20 a of a VA mode liquidcrystal panel. Formed on the active matrix substrate 20 a are: aplurality of scanning lines 23 extending substantially in parallel toone another; a plurality of signal lines 24 extending in a directionintersecting the scanning lines 23; TFTs 25 electrically connected tocorresponding scanning lines 23 and signal lines 24; and pixelelectrodes 26 electrically connected to the TFTs 25. Each TFT 25 andeach pixel electrode 26 are provided in each one of a plurality of pixelregions arranged in a matrix array. On the active matrix substrate 20 a,storage capacitor lines 23′ for composing storage capacitors are furtherformed.

On the surface of the active matrix substrate 20 a shown in FIG. 3, analignment film 22 a having vertical alignment properties is formed. Thealignment film 22 a has been subjected to a predetermined alignmenttreatment, such that the alignment film 22 a defines the pretilt angleand pretilt direction of the liquid crystal molecules. Note that the“pretilt angle” is an angle between the major axis of a liquid crystalmolecule whose orientation is regulated by the orientation regulatingforce of the alignment film surface and the substrate surface. The“pretilt direction” is an azimuthal direction of the major axis of aliquid crystal molecule whose orientation is regulated by theorientation regulating force of the alignment film surface. Since thepretilt direction of a liquid crystal molecule is defined by theorientation regulating force of an alignment film, the direction of theorientation regulating force of an alignment film is also expressed bythe term “pretilt direction” in the present specification. Asillustrated with respect to a lower left pixel in FIG. 3, the alignmentfilm 22 a has different pretilt directions (solid arrows in the figure)respectively for four regions within the pixel region.

Also, an alignment film 22 b having vertical alignment properties isformed on the surface of a color filter substrate 20 b opposing theactive matrix substrate 20 a. As shown in FIG. 3, the alignment film 22b has different pretilt directions (dotted arrows in the figure)respectively for four regions within the pixel region. As shown in FIG.3 and FIG. 4, these pretilt directions are set so as to be opposite tothe pretilt directions of the alignment film 22 a on the active matrixsubstrate 20 a side.

In the VA mode liquid crystal display device, liquid crystal molecules21 a contained in the liquid crystal layer 21 have a negative dielectricanisotropy such that, under an applied voltage, the liquid crystalmolecules 21 a having a negative dielectric anisotropy will be tiltedfrom a substantially vertical state. Since the pretilt directions of thealignment films 22 a and 22 b are set in the above-described manner,under an applied voltage, the liquid crystal layer 21 will form fourliquid crystal domains characterized by different orientation directionsof the liquid crystal molecules 21 a. In other words, each pixel regionis orientation-divided into four regions in which liquid crystalmolecules will tilt in different directions (four-divided orientation).As a result of this, the viewing angle dependence of displaying isreduced, whereby the viewing angle characteristics are improved.

Next, an IPS mode liquid crystal display device will be described. FIG.5 schematically shows an active matrix substrate 20 a of an IPS modeliquid crystal panel. Formed on the active matrix substrate 20 a are: aplurality of scanning lines 23 extending substantially in parallel toone another; a plurality of signal lines 24 extending in a directionintersecting the scanning lines 23; TFTs 25 electrically connected tocorresponding scanning lines 23 and signal lines 24; and pixelelectrodes 26 electrically connected to the TFTs 25. The pixelelectrodes 26 are formed in the shape of combteeth extendingsubstantially in parallel to the signal lines 24.

On the active matrix substrate 20 a, common electrodes are furtherprovided, which are formed in the shape of combteeth that aresubstantially parallel to the pixel electrodes 26. The common electrodes27 extend from common lines 28, which are formed substantially inparallel to the scanning lines 23. Via an insulative film (not shown),the common lines 28 oppose storage capacitor electrodes 29, which areformed of the same conductive layer as the pixel electrodes 26, and thusconstitute storage capacitors.

On the surface of the active matrix substrate 20 a shown in FIG. 5, analignment film 22 a having horizontal alignment properties is formed.Also, on the surface of the color filter substrate 20 b opposing theactive matrix substrate 20 a, an alignment film 22 b having horizontalalignment properties is formed.

In an IPS mode liquid crystal display device, liquid crystal moleculescontained in the liquid crystal layer 21 have a positive dielectricanisotropy such that, under an applied voltage, their orientationdirections are changed by lateral fields which are generated between thepixel electrodes 26 and the common electrodes 27 (electric fields whichare parallel to the layer plane of the liquid crystal layer). In an IPSmode liquid crystal display device, good viewing angle characteristicsare realized because the orientation directions of the liquid crystalmolecules vary within the plane which is parallel to the liquid crystallayer 21.

Note that the IPS mode has a problem in that a coloring phenomenonoccurs when observed in an oblique direction (a direction which istilted from the substrate-plane normal direction). Specifically, thelight becomes bluish when observed in the longitudinal direction of theliquid crystal molecules, whereas the light becomes yellowish whenobserved in the minor-axis direction of the liquid crystal molecules. Inother words, the light passing through the liquid crystal layer in anoblique manner (in a direction tilted from the layer normal direction)may become bluish or yellowish. This is because retardation of theliquid crystal molecules has a wavelength dispersion (wavelengthdependence).

In order to suppress the aforementioned coloring phenomenon, aconstruction as shown in FIG. 6 may be adopted. An active matrixsubstrate 20 a shown in FIG. 6 includes signal lines 24 which are bent aplurality of times (zigzag-shaped), as well as pixel electrodes 26 andcommon electrodes 27 which are bent so as to be substantially parallelto the signal lines 24 (in the “<” shape).

Since the pixel electrodes 26 and the common electrodes 27 have bentshapes as described above, under an applied voltage, two regionscharacterized by different orientation directions of the liquid crystalmolecules are created in each pixel region. Therefore, when observed ina certain oblique direction, each region causes the wavelength region oflight to be shifted to a hue of a complementary color, whereby thecoloring phenomenon is suppressed.

Next, low-viscosity liquid crystal materials containing molecules whichare unstable against ultraviolet light will be specifically described.Note that a liquid crystal material is generally a mixture of aplurality of types of molecules (compounds), and any molecule composingthe liquid crystal material may not necessarily exhibit liquid crystalproperties as a simple substance.

When molecules having at least one of a carbon-carbon triple bond and apolycyclic group are mixed in a liquid crystal material, the viscosityof the liquid crystal material is lowered, whereby the responsecharacteristics of the liquid crystal display device can be improved.Although molecules having at least one of a carbon-carbon triple bondand a polycyclic group have low stability against ultraviolet light, thepresent invention suppresses decomposition of such molecules, whereby adecrease in the voltage retention rate and display unevenness can beprevented. In particular, when using a liquid crystal material whosecoefficient of rotational viscosity γ₁ at approximately 20° C. is about120 mPa·s or less, there is a large significance in using preferredembodiments of the present invention because decrease in the voltageretention rate and display unevenness are likely to occur. Note that, inthe present specification, “polycyclic groups” refer to bothnon-condensed polycyclic groups and condensed polycyclic groups.

Examples of molecules having at least one of a carbon-carbon triple bondand a polycyclic group include molecules having a chemical structureexpressed by any of the following formulae. By mixing such moleculesinto the liquid crystal material, it can be easily ensured that thecoefficient of rotational viscosity γ₁ of the liquid crystal material atapproximately 20° C. is about 120 mPa·s or less. Note that n in thefollowing formulae is an integer equal to or greater than 2, and anyhydrogen atom contained in a ring structure in the following formulaemay independently be substituted by a halogen atom, a cyano group, or anisocyano group.

By mixing about 25 weight % or more of molecules having any suchchemical structure into the liquid crystal material, the viscosity ofthe liquid crystal material is sufficiently lowered, whereby rapidresponse can be obtained. Specifically, a response time of about oneframe or less can be realized, and a level of moving picture performancethat is required of a liquid crystal television set can be obtained.

Among molecules having the aforementioned chemical structures, moleculeshaving a tolan group (i.e., molecules including structures expressed bythe formulae shown by [formula 5] below, specific examples beingmolecules expressed by formulae (I) and (VI)) provide greatviscosity-reducing effects, and yet have very low stability againstultraviolet due to their triple bonds. Thus, the effects of preferredembodiments of the present invention will be most clearly exhibited forthem.

Hereinafter, examples of liquid crystal materials and their constituentmolecules will be described more specifically.

As low-viscosity liquid crystal materials, liquid crystal materialscontaining molecules expressed by formula (I) below can be used, forexample. In formula (I), m and n are integers equal to or greaterthan 1. Liquid crystal materials containing molecules as expressed byformula (I) are disclosed in IDW '00, p. 77, for example, and can have acoefficient of rotational viscosity γ₁ of about 111 to about 114 mPa·sat 20° C.

Alternatively, liquid crystal materials containing molecules which areexpressed by formula (II) below can be used. In formula (II), each of Aand B is independently a cyclohexylene, a phenylene, a phenylene some ofwhose H's are substituted by F's, or a cyclohexylene at least one ofwhose H's is substituted by D; at least one of Z₁ and Z₂ is —C≡C—; R1 isan alkyl, an alkenyl, an oxaalkyl, or an alkoxy (where preferably thenumber of C's is no less than 1 and no more than 10); and X₁, X₂, and X₃are H or F. Typically, X₂ is F, and at least one of X₁ and X₃ is F.

Liquid crystal materials containing molecules as expressed by formula(II) are disclosed in Japanese Laid-Open Patent Publication No.10-292173, for example, and can have a coefficient of rotationalviscosity γ₁ of about 28 mPa·s or less at 20° C. Molecules expressed byformula (II) include structures expressed by the following formulae, forexample.

Alternatively, liquid crystal materials containing molecules expressedby formulae (III), (IV) and (V) below can be used. In formulae (III),(IV), and (V), R is an alkyl, an alkenyl, an oxaalkyl, or an alkoxy;each of X₄, X₂, X₃, and X₄ is, independently, H or F; and Y is F, —CF₃,—OCF₃, —OCHF₂, —OCH₂F, or R. Liquid crystal materials containingmolecules as expressed by formulae (III), (IV), and (V) are disclosed inJapanese Laid-Open Patent Publication No. 2002-38154, for example.

Moreover, liquid crystal materials containing molecules expressed byformula (VI) can be used for an IPS mode liquid crystal display device(having an active matrix substrate 20 a as shown in FIG. 5 or FIG. 6,for example). In formula (VI), m and n are integers equal to or greaterthan 1.

Liquid crystal materials containing molecules as expressed by formula(VI) are disclosed in Japanese Laid-Open Patent Publication No.7-316556, for example. As is disclosed in this publication as Example 3,a liquid crystal material in which molecules expressed by formula (VI)and molecules expressed by formula (VII) are mixed has a coefficient ofrotational viscosity γ₁ of about 20 mPa·s at approximately 20° C.

Furthermore, liquid crystal materials containing molecules expressed byformula (VIII), (IX), and (X) can be used for a VA mode liquid crystaldisplay device (having an active matrix substrate 20 a as shown in FIG.3, for example). In formula (VIII), (IX), and (X), each of X₁ to X₆ is,independently, a hydrogen atom, a halogen atom, a cyano group, or anisocyano group. However, it is preferable that at least one of X₁, X₂and X₃, at least one of X₄ and X₅, and X₆ are not hydrogen atoms.Moreover, those of X₁ to X₆ which are not hydrogen atoms are preferablyhalogen atoms, and more preferably fluorine atoms.

Liquid crystal materials containing molecules as expressed by formula(VIII), (IX), and (X) are disclosed in Japanese Laid-Open PatentPublication No. 2002-69449, for example. A liquid crystal material whichis disclosed in this publication as Example 1 has a negative dielectricanisotropy, and can be used for a VA mode liquid crystal display device.

Next, results of evaluating the reliability of actually-produced liquidcrystal display devices will be described. The inventor has actuallyproduced liquid crystal display devices each including a liquid crystalpanel having a liquid crystal layer which is formed of a low-viscosityliquid crystal material and an illuminator including light sourcescausing primary generation of light to be used for displaying, andevaluated their reliabilities.

First, the VA mode active matrix substrate 20 a shown in FIG. 3 and thecolor filter substrate 20 b were produced by known techniques. On thesurfaces of the active matrix substrate 20 a and the color filtersubstrate 20 b, an alignment film material whose main structure ispolyimide and which has a side chain that induces vertical alignmentproperties as well as a photoreactive side chain composed of a chalconegroup was applied so as to form alignment films, and these alignmentfilms were irradiated with polarized ultraviolet light from a directionoblique to the substrate-plane normal direction. The active matrixsubstrate thus produced was attached to the color filter substrate, anda liquid crystal material was injected into the gap therebetween, thusproducing a liquid crystal panel. As the liquid crystal material, aliquid crystal material containing molecules having a naphthalene groupwas used.

A plurality of liquid crystal panels as described above were produced,and on the rear surfaces of these liquid crystal panels, illuminators #1to #4 having red LEDs, green LEDs and blue LEDs were provided, thusproducing liquid crystal display devices (Prototypes 1 to 4). Moreover,illuminator #5 having a cold-cathode tube (CCFL) was provided on therear surface of a liquid crystal panel as described above, thusproducing a liquid crystal display device (Comparative Example 1). Theemission spectra of blue LEDs #1 to #4 used for illuminators #1 to #4are shown in FIG. 7 to FIG. 10, whereas the emission spectrum of thecold-cathode tube (CCFL) used for illuminator #5 is shown in FIGS. 11(a) and (b). Note that FIG. 11( b) is a graph obtained by magnifying thevertical axis of FIG. 11( a) by 10 times. The peak wavelengths of blueLEDs #1 to #4 are shown in Table 1.

TABLE 1 LED #1 LED #2 LED #3 LED #4 peak 365 382 405 465 wavelength (nm)

The liquid crystal display devices of Prototypes 1 to 4 and the liquidcrystal display device of Comparative Example 1 were observed withrespect to aging. However, in order to conduct accelerated tests, theluminance of the light sources was set so as to be 10 times as large asthe usual luminance.

In the liquid crystal display devices of Prototypes 1 to 4, no changesoccurred after 500 hours. However, in the liquid crystal display deviceof Comparative Example 1, changes began to occur in the orientationdirections (pretilt directions) after 500 hours, and a decrease in thevoltage retention rate was also observed.

Moreover, in the liquid crystal display device of Comparative Example 1,changes in the orientation directions became greater after 1000 hours,and conspicuous display unevenness was observed. On the other hand,among the liquid crystal display devices of Prototypes 1 to 4, a slightdecrease in the voltage retention rate was observed for Prototype 1, butno change was observed for Prototypes 2 to 4.

The changes in the orientation directions and decrease in the voltageretention rate in the liquid crystal display device of ComparativeExample 1 are ascribable to the ultraviolet light which is generated bythe cold-cathode tube of illuminator #5. As shown in FIGS. 11( a) and(b), the emission spectrum of the cold-cathode tube exhibits peaks at313 nm (j line) and 365 nm (i line). These peaks correspond to emissionlines that are characteristic of mercury emission, and are present inthe emission spectrum of a cold-cathode tube due to its principles.These emission lines cause deterioration of the molecules, and thuslower the reliability.

The light near the 313 nm peak, in particular, contributes much to thedecomposition of the molecules in the liquid crystal layer. The reasonthereof is described below.

Absorption wavelength bands of a carbon-carbon (C—C) bond, acarbon-hydrogen (C—H) bond, and benzene are toward theshorter-wavelength side from 300 nm. Therefore, these bonds are unlikelyto be severed by mercury emission. However, if any conjugated systemexists in the molecules, the absorption wavelength will shift toward thelonger-wavelength side. Moreover, the amount of shift will depend on thenumber and length of conjugated systems. For example, shifts toward thelonger-wavelength side may occur as follows: benzene has an absorptionwavelength of 261 nm, whereas naphthalene has that of 312 nm andanthracene has that of 375 nm.

In the case of compounds that have significant viscosity-reducingcharacteristics for improving the response characteristics of the liquidcrystal display device, i.e., compounds having a conjugated system suchas a naphthalene group, a biphenyl group, or a carbon-carbon triple bond(already described), the absorption wavelength also shifts to 300 nm ormore. Therefore, out of the light which is generated by mercury, it isthe 313 nm light (j line) that is most influential on the decompositionof molecules, followed by the 365 nm light (i line).

In the case of a compound which causes a large amount of shift and hasan absorption peak near 365 nm (i line), for example, the 365 nm (iline) light is the most influential light. In this case, however, theabsorption edge reaches the visible region so that blue light will beslightly absorbed. As a result, the compound itself will becomeyellowish, which is not favorable from the standpoint of displaycharacteristics. After all, in a liquid crystal material in which bothviscosity and hue are optimized, it is the 313 nm light (j line) that ismost influential on the decomposition of molecules, followed by the 365nm light (i line).

On the other hand, blue LEDs #1 to #4 cause primary generation of bluelight, and thus the emission spectra of blue LEDs #1 to #4 do not have apeak at least near 313 nm, as shown in FIG. 7 to FIG. 10. Therefore, thelight which is generated by blue LEDs #1 to #4 is unlikely to decomposethe molecules in the liquid crystal layer.

As described above, it has been confirmed that the reliability of aliquid crystal display device comprising a liquid crystal layer which isformed of a low-viscosity liquid crystal material is improved by usingan illuminator that includes light sources causing primary generation ofat least blue light, among other light which is used for displaying.

Note that, as can be seen from the fact that a slight decrease in thevoltage retention rate was observed in Prototype 1 after 1000 hours,from the standpoint of further improving the reliability, it ispreferable that the blue light which is generated by the light sourceshas a spectrum such that its peak wavelengths are at 380 nm or more(i.e., so as to fall within the visible region), as is the case withblue LEDs #2 to #4 of Prototypes 2 to 4. Moreover, it is more preferablethat the peak wavelengths are at 400 nm or more as is the case with blueLEDs #3 and #4, and it is further preferable that substantially no lightin the ultraviolet region is generated as is the case with blue LED #4.The reason is that, since the absorption edge of an organic compound hasan extent (i.e., the absorption spectrum has a wide breadth), evenultraviolet light in a region close to the visible region (i.e., on thehigher-wavelength side of the j line and the i line) will slightlycontribute to decomposition of molecules, and will be cumulated duringhours of use of the liquid crystal television set (e.g., 40000 hours) toexhibit an influence.

Next, the active matrix substrate 20 a for the IPS mode shown in FIG. 5and the color filter substrate 20 b were produced by known techniques.On the surfaces of the active matrix substrate 20 a and the color filtersubstrate 20 b, an alignment film material having horizontal alignmentproperties (i.e., causing hardly any pretilt) was applied so as to formalignment films, and these alignment films were irradiated withpolarized ultraviolet light from the substrate-plane normal direction.The active matrix substrate thus produced was attached to the colorfilter substrate, and a liquid crystal material was injected into thegap therebetween, thus producing a liquid crystal panel. As the liquidcrystal material, a liquid crystal material containing molecules havinga tolan group was used.

A plurality of liquid crystal panels as described above were produced,and on the rear surfaces of these liquid crystal panels, illuminators #1to #4 having red LEDs, green LEDs and blue LEDs were provided, thusproducing liquid crystal display devices (Prototypes 5 to 8). Moreover,illuminator #5 having a cold-cathode tube (CCFL) was provided on therear surface of a liquid crystal panel as described above, thusproducing a liquid crystal display device (Comparative Example 2).

The liquid crystal display devices of Prototypes 5 to 8 and the liquidcrystal display device of Comparative Example 2 were observed withrespect to aging. However, in order to conduct accelerated tests, theluminance of the light sources was set so as to be 10 times as large asthe usual luminance.

In the liquid crystal display device of Prototypes 5 to 8, no changesoccurred after 500 hours. However, in the liquid crystal display deviceof Comparative Example 2, changes began to occur in the orientationdirections (pretilt directions) after 500 hours, and a decrease in thevoltage retention rate was also observed.

Moreover, in the liquid crystal display device of Comparative Example 2,changes in the orientation directions became greater after 1000 hours,and conspicuous display unevenness was observed. On the other hand,among the liquid crystal display devices of Prototypes 5 to 8, a slightdecrease in the voltage retention rate was observed for Prototype 5, nochange was observed for Prototypes 6 to 8.

As described above, it has been confirmed that the reliability of an IPSmode liquid crystal display device including a liquid crystal layerwhich is formed of a low-viscosity liquid crystal material is improvedby using an illuminator that includes light sources causing primarygeneration of at least blue light, among other light which is used fordisplaying.

Note that preferred embodiments of the present invention are applicableto liquid crystal display devices of various display modes, and may beused for a TN mode liquid crystal display device, for example, withoutbeing limited to the VA mode and IPS mode described above.

However, the effects of improving reliability were higher for the VAmode than for the TN mode. The reason thereof will be described withreference to FIG. 12 to FIG. 15. FIG. 12 and FIG. 13 are graphs showingvoltage-transmittance curves of a VA mode liquid crystal display device.FIG. 14 and FIG. 15 are graphs showing voltage-transmittance curves of aTN mode liquid crystal display device. The five curves shown in FIG. 12and FIG. 13 indicate, from the uppermost curve, cases where the pretiltangles are 87.9°, 88.4°, 88.9°, 89.4°, and 89.9°. The five curves shownin FIG. 14 and FIG. 15 indicate, from the uppermost curve, cases wherethe pretilt angles are 0.1°, 0.6°, 1.1°, 1.6°, and 2.1°.

As can bee seen from a comparison between FIGS. 12 and FIGS. 14 and 15,and in particular from a comparison between FIG. 13 and FIG. 15 wherethe transmittance is shown in logarithm on the vertical axis, thevoltage-transmittance curves at the black level to low-luminance grayscale levels (i.e., portions surrounded by broken lines in FIG. 13 andFIG. 15) are steeper and the amount of change in transmittance withrespect to changes in the pretilt angles is greater in the VA mode thanin the TN mode. Note that the gray scale level has an exponentialrelationship with transmittance. For example, the transmittance T_(n) atan n^(th) gray scale level in 256 gray scale-level displaying isexpressed as T_(n)=(n/255)^(2.2). Therefore, in order to discuss therelationship between gray scale levels and transmittance, it ispreferable to employ semi-logarithmic plotting as in FIG. 13 and FIG.15.

Since the amount of change in transmittance with respect to changes inthe pretilt angles is greater in the VA mode as mentioned above, in theVA mode, display unevenness may occur even if slight changes occur inthe pretilt angles due to decomposition of the molecules in the liquidcrystal layer. Therefore, the reliability-improving effects achieved bypreferred embodiments of the present invention are high. Moreover, inthe case where alignment division is adopted, changes in the pretiltangles may cause changes in the positions of the boundaries betweendomains, whereby displaying coarseness may be observed. Therefore, thereliability-improving effects are especially high in a VA mode whereorientation division is adopted.

Moreover, preferred embodiments of the present invention provide highreliability-improving effects also in the IPS mode. In the IPS mode,displaying is performed by generating lateral fields usingcombteeth-like electrodes. However, since no lateral fields occur abovethe electrodes, the portions where the electrodes are formed do notcontribute to displaying. Therefore, the effective aperture ratio islower than that in the TN mode or the VA mode, and is typically abouthalf of that in the TN mode or the VA mode. For this reason, in order toobtain the same luminance as in the TN mode or the VA mode, it isnecessary to increase the brightness of the light sources to abouttwice. If an illuminator including a cold-cathode tube is employed as inthe conventional case, decomposition of the molecules in the liquidcrystal layer is likely to occur. Hence, the reliability-improvingeffects of the present invention are high.

Furthermore, the reliability-improving effects achieved by preferredembodiments of the present invention will also be clear in an FFS(fringe field switching) mode where, as in the IPS mode, the alignmentstate of a horizontal alignment type liquid crystal layer is controlledby using lateral fields.

Preferred embodiments of the present invention are suitably used in apassive matrix-type liquid crystal display device or an activematrix-type liquid crystal display device, but provides clear effectsespecially in an active matrix-type liquid crystal display device. In anactive matrix-type liquid crystal display device where a switchingelement (e.g., a TFT) is provided in each pixel, the charge which ischarged in the pixel capacitance must be retained during one frame. Ifthe molecules in the liquid crystal layer are decomposed by ultravioletlight, the voltage retention rate will decrease, thus resulting in alower display quality. According to preferred embodiments of the presentinvention, such a decrease in the voltage retention rate can besuppressed, and therefore active matrix driving can be performed in afavorable manner.

Note that ultraviolet light is also contained in external light enteringthe liquid crystal panel 20, and the light which is generated by blueLEDs may also contain a slight amount of light in the ultravioletregion. Therefore, in order to more certainly suppress decomposition ofmolecules due to ultraviolet light, members for absorbing ultravioletlight may be provided at the illuminator side or the viewer side of theliquid crystal layer 21, or members positioned at the illuminator sideand the viewer side of the liquid crystal layer 21 may be formed from amaterial which absorbs ultraviolet light.

However, in a liquid crystal display device incorporating an illuminatorwhich includes a cold-cathode tube, decomposition of the molecules inthe liquid crystal layer will occur even if members for absorbingultraviolet light are provided. Polarizing plates having TAC (triacetylcellulose) films, which contain an ultraviolet absorber, were used inthe aforementioned Prototype- and Comparative-Example liquid crystaldisplay devices, but decomposition of molecules nonetheless occurred inthe Comparative Examples. This is because even a member which absorbsultraviolet light cannot absorb all of the ultraviolet light that isgenerated upon light emission due to its principles.

FIG. 16 shows an absorption spectrum of a TAC film containing anultraviolet absorber. As can be seen from FIG. 16, this TAC film hasabsorptivity with respect to light of a wavelength of 400 nm or less.However, its OD (Optical Density) value is about 1 to 4, and thus it isnot able to completely absorb ultraviolet light. Therefore, evenultraviolet light which is so feeble that it cannot be detected by anilluminometer may, when radiated onto the liquid crystal layer for longhours, reach a point where its cumulative energy decomposes themolecules.

Although the preferred embodiments described herein illustrate LEDs aslight sources, this is not a limitation. Any light source can be broadlyused which cause primary generation of at least blue light. For example,electroluminescence (EL) elements can be used. Note that LEDs maysometimes be referred to as EL elements (EL element in the broad sense)because they perform light emission by utilizing electroluminescence.However, in the present specification, “EL elements” refer to intrinsicEL elements such as so-called organic EL elements and inorganic ELelements, and do not refer to injection-type EL elements such aslight-emitting diodes (LEDs), unless otherwise specified. An illuminatorthat includes red EL elements, green EL elements, and blue EL elementsmay be used, or an illuminator that includes blue EL elements andphosphors which absorb light from the blue EL elements and generatelight in longer wavelength regions may also be used. Alternatively, anilluminator that includes white EL elements in which red, green, andblue emission layers are overlaid may be used.

Moreover, it is even possible to use discharge tubes which do not causeprimary generation of ultraviolet light, such as neon tubes enclosing anoble gas causing primary generation of light which is used fordisplaying. For example, since a neon tube enclosing neon is capable ofprimary generation of fire red and an argon tube enclosing argon iscapable of primary generation of blue-green light, a white light sourcecan be obtained by combining a neon tube, an argon tube, and a colorfilter for adjusting the color balance, for example.

According to preferred embodiments of the present invention, thereliability of a liquid crystal display device including a liquidcrystal layer which is formed of a low-viscosity liquid crystal materialcan be improved, and a liquid crystal display device which is capable ofperforming high-quality displaying for long hours is provided.

A liquid crystal display device according to preferred embodiments ofthe present invention can be suitably used for various electronicapparatuses which are expected to be used for long periods of time. Forexample, it can be suitably used for a liquid crystal television setwhich includes circuitry for receiving television broadcasts.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following.

1. A liquid crystal display device comprising: an illuminator; and aliquid crystal panel arranged to perform a display operation using lightwhich is emitted from the illuminator; wherein: the liquid crystal panelincludes a pair of substrates and a horizontal alignment type liquidcrystal layer provided between the pair of substrates; the liquidcrystal panel controls an alignment state of the horizontal alignmenttype liquid crystal layer by using lateral fields; the liquid crystallayer is formed of a liquid crystal material which contains moleculeshaving at least one of a carbon-carbon triple bond and a polycyclicgroup; the illuminator includes a light source causing primarygeneration of at least blue light, among other light which is used fordisplaying; and the liquid crystal panel performs displaying in anin-plane switching mode or a fringe field switching mode.
 2. The liquidcrystal display device of claim 1, wherein a coefficient of rotationalviscosity γ₁ of the liquid crystal material at 20° C. is 120 mPa·s orless.
 3. The liquid crystal display device of claim 1, wherein themolecules contained in the liquid crystal material have a chemicalstructure expressed by one of the following formulae:

(where n in the formulae is an integer equal to or greater than 2; andany hydrogen atom contained in a ring structure in the formulae may be,independently, substituted by a halogen atom, a cyano group, or anisocyano group).
 4. The liquid crystal display device of claim 1,wherein the liquid crystal panel includes a polarizing plate including aTAC film having absorptivity with respect to light of a wavelength of400 nm or less.
 5. The liquid crystal display device of claim 1, whereinthe light source generates substantially no light in an ultra-violetregion.
 6. The liquid crystal display device of claim 1, wherein thelight source is an electroluminescence element.
 7. The liquid crystaldisplay device of claim 1, wherein the liquid crystal layer includesliquid crystal molecules having a positive dielectric anisotropy.
 8. Theliquid crystal display device of claim 3, wherein the liquid crystalmaterial contains 25 weight % or more of the molecules having thechemical structure.
 9. The liquid crystal display device of claim 1,wherein: one of the pair of substrates includes a pixel electrode and acommon electrode; and the pixel electrode has a bent shape.
 10. Theliquid crystal display device of claim 9, wherein: the pixel electrodeis formed in the shape of combteeth; and at least a part of the pixelelectrode is parallel to at least a part of the common electrode.